1. Field of the Invention
The present application relates to a laser apparatus comprising a semiconductor laser diode (LD) and a bypassing capacitor disposed in a vicinity of the LD.
2. Background Arts
In an optical communication system, an advanced technique to utilize a phase of signal light has become popular to bring further communication capacity. Such an optical communication system is often called as the coherent communication system. FIG. 7 schematically illustrates an arrangement of an optical signal source 100 used in the coherent communication system. The optical signal source 100 shown in FIG. 7 provides an LD 102 biased with a DC power supply 101 and an optical modulator 103. The LD 102 emits continuous-wave (CW) light L11, and the optical modulator 103 modulates the CW light L11 to output a modulated light L12. The optical signal source 100 sometimes installs a wavelength tunable LD as the LD 102 disclosed in, for instance, the U.S. Pat. No. 7,362,782.
The LD 102 implemented in the optical signal source 100 is strongly requested to generate the CW light with line width thereof as narrower as possible. Electrical noises superposed on the bias provided to the LD 102 degrade the line width. The DC power supply 101 inherently causes noises, but the bias line 104 sometimes superposes noises by the electro-magnetic interference (EMI), in particular, noises with high frequencies. A bias line for supplying a DC bias usually accompanies with bypassing capacitors against the ground. However, the bypassing capacitor is necessary to be connected to the LD as close as possible because, when a substantial bias line is left between the LD and the bypassing capacitor, the left bias line causes noises. In particular, when the optical modulator 103 modulates the CW light L11 by modulation signals whose frequency reaches and sometimes exceeds 10 GHz, the left bias line between the LD and the bypassing capacitor is further preferable as short as possible.